Medical elastic bands may be used for a plurality of procedures and tasks, for example, to supply force to orthodontic appliances and to ligate tissue. Medical elastic bands are commonly small and possess a relatively thicker material portion than other common elastic bands (e.g., common rubber bands). The inside diameter of a relaxed band is commonly only one-third of the band outer diameter. For at least these exemplary tasks, medical elastic bands act as energy storage devices. Specifically, energy is stored via the stretching of a free state elastic band, and the stored energy is "released" as the band contracts. For medical elastic bands required to have a long shelf life, it is important that, even near the end of such shelf life, that the elastic bands be capable of releasing maximum stored energy.
In ligation procedures, elastic bands are used to prevent fluid flow through ligated tissue. Ligating bands are typically dispensed from a ligating band dispenser, which may be fixed to the distal end of a hosting instrument, for example, an endoscope or ligating instrument. Ligating band dispensers are typically cylindrical and hollow in nature, where an inner diameter of the dispensers defines a cavity and an outer diameter carries one or more expanded ligating bands. Dispensers include a dispensing mechanism, allowing a user to remotely cause the dispensing of one or more ligating bands from the dispenser.
For a ligation procedure, a ligating band dispenser, mounted on an appropriate instrument, is positioned adjacent tissue targeted for ligation. Means are used to draw the tissue into the cavity defined by the inner diameter of the dispenser. These means may include suction, provided through a suction lumen which opens into the cavity, or a grasping device to grasp and physically draw tissue into the cavity region. Once the targeted tissue is properly positioned, the dispensing mechanism is actuated in such a manner so as to cause the dispensing of at least one ligating band. In the instant the ligating band is discharged from the dispenser, the dispensed ligating band attempts to return to its original, unexpanded dimensions, thus effectively ligating the targeted tissue. Subsequently, the means used to draw the tissue into the cavity region is ceased or disengaged, and the dispenser is moved away from the ligated tissue.
Where a ligating band is placed about, for example, a ballooning varix, polyp, hemorrhoids, or pre-cancerous lesion, the contracted ligating band induces fusion and healing in the base tissue and subjects the ligated tissue to necrosis. The necrotic tissue eventually separates from the surrounding tissue and simply passes into the human system. Alternatively, ligation may also be used for purposes of sterilization, wherein a ligating band may be placed over a folded loop portion of a Fallopian tube or a vas deferens to prevent the passage of internal reproductive fluids.
Conventional ligating bands (see FIG. 1) are formed from a highly elastic, homogenous elastomer material and typically assume a toroidal shape. Conventional bands for ligation of esophageal varices, as an example, have a relaxed inner diameter of approximately 1.8 mm and an outer diameter of 5.3 mm. When circumferentially expanded, these bands transition, as a whole, from neutral stress to tension. Circumferential expansion is limited to approximately seven times the bands' relaxed dimensions based on elastomer elongation strain limits.
Inherent to the shaping of these bands, the inner diameter of conventional ligating bands, being significantly smaller than the outer diameter, experiences significantly higher levels of strain, and therefore, stress, than an outer diameter when circumferentially expanded. Band material stretches and thins during elongation to a diameter many times the relaxed dimensions.
The multiple of strain and stress experienced by the inner diameter relative to the outer diameter of conventional ligating bands is approximately the ratio of the outer diameter to the inner diameter. In an expanded condition, the inner and outer diameters are nearly the same, or a ratio of one to one. In an unexpanded condition, the outer diameter is three times the inner diameter. For these bands, the strain and stress at the inner diameter will be three times greater than at the outer diameter when expanded. In other words, when the inner diameter of this conventional ligating band elongates seven times its relaxed dimensions (a maximum condition), for example, the outer diameter typically elongates less than three times its relaxed diameter. Accordingly, the inner diameter material is subjected to higher stresses than that of the outer diameter. This distribution of stresses is a limiting factor for enhancing elongation and ligation performance.
A further observation regarding the expansion characteristics of conventional ligating bands, the elongation of material at the outer diameter is typically less than one-half of its maximum elongation capability when the band is in a fully expanded condition. Conventional ligating bands therefore store less than maximum energy to ligate tissue, thus inefficiently utilizing the elastic material from which it is formed. To this end, larger bands of conventional construction are required to deliver the same degree of constriction as bands which maximize their energy storage potential.
Referring again the operational performance of conventional ligating bands in the context of stress and strain, in a fully expanded state, conventional ligating bands have a ratio of inner diameter to outer diameter of approximately 1:1. For this fully expanded state, the inner diameter of a conventional ligating band elongates approximately seven times its relaxed dimensions as compared to the outer diameter elongating less than three times its relaxed diameter. Consequently, the limited elongation of the outer diameter reveals that a majority of material near the outer diameter of a conventional band is utilized at less than 50% of its energy storage potential. In other words, while the inner diameter of a conventional ligating band fully utilizes its energy potential when expanded, such use drops to approximately 30% at the outer diameter of the band. Continued circumferential expansion following an inner diameter reaching a maximized energy storage potential can result in damage to the ligating band or breakage as the inner diameter becomes over-stressed.
As provided above, conventional ligating bands for ligation of esophageal varices have an original, relaxed inner diameter of approximately 1.8 mm. Conventional ligating bands are commonly pre-loaded and stored on a ligating band dispenser. Ligating band dispensers typically have an outer diameter of approximately 12.7 mm--representing a maximum stretched condition for conventional bands. Ligating bands may remain in this fully stretched condition for up to their full shelf life, for example, two years.
The duration of time in which the stored ligating bands are subjected to maximized internal stresses can result in permanent stretch, set, creep, or flow of material. While conventional ligating bands are originally incapable of ligating tissue having a cross-sectional area less than 2.54 mm.sup.2, this area likely increases for ligating bands stored for extended periods on a ligating band dispenser. Given the relationship between stresses of the inner diameter and the outer diameter of conventional ligating bands (as well as the limitations of conventional materials), efforts to enhance ligating performance on smaller tissues through the reduction of the original, relaxed inner diameter results in a disproportionate reduction in the operational (or expansion) range of the ligating bands. Importantly, any such reduction in operational range would likely preclude such ligating bands from being used with conventional ligating band dispensers.